Dry eye syndrome (or keratoconjunctivitis sicca (KCS)) is the name given to the condition characterized by a lack of the required quantity or composition of tears to properly lubricate the eyes. It is a relatively common condition that increases in frequency with age. Dry eye syndrome affects roughly twice as many women as it does men. Approximately 9 million Americans suffer from the condition.

Tears are vital to the healthy functioning of the eye. They assist in keeping the eye comfortable and the vision optimal. The film of tears that do this job are composed of three layers. The innermost layer is made of mucus which is produced by and covers the clear skin that lines the eye (conjunctiva). The middle layer is composed of a diluted saltwater solution. This layer serves to keep the eye moist and comfortable, as well as flushing out foreign objects like dust. The outermost layer is composed of lipids (fats and oils) and primarily serves to protect the middle layer.

Dry eye syndrome is usually caused as a result of a defect in the middle or aqueous layer of the tear film which leads to either decreased tear production or increased tear evaporation.